The demand for printed wiring boards having electronic components such as IC (integrated circuits) and LSI (large scale integrated circuits) mounted thereon is rapidly increasing in accordance with the progress of electronic industry.
In the production of the printed wiring boards, kraft paper, glass cloth, glass nonwoven fabric or the like are impregnated with a thermosetting resin such as a phenolic resin or an epoxy resin to obtain a pre-preg. This pre-preg and a copper foil are bonded with each other by, for example, hot pressing. Thereafter, resist printing and masking film lamination are used to form circuit patterns. Unwanted portions of the copper foil are etched away with the use of an acid or an alkali to form a desired circuit pattern, and the resist and masking film are removed. After the formation of the desired circuit pattern, electronic components are set at the predetermined positions of the printed wiring board and dipped in a solder bath so that the electronic components are fixed on the printed wiring board.
Two types of copper foils, namely electrodeposited copper foil and rolled copper foil, are available for use in printed wiring boards. These days, however, electrolytic copper foil is more often employed because of its wide applicability and because of the ease and low cost in forming a thinner copper foil.
Electrodeposited copper foil for use in printed wiring boards is conventionally produced through the following process.
That is, a copper sulfate solution is placed in a electrolyzing bath and anodes composed of insoluble electrodes, are disposed in the electrolyzing bath. Furthermore, a rotating cathode drum is disposed in the electrolyzing bath so that almost half of the drum is immersed in the copper sulfate solution and the surface of the drum is opposite to the anodes. Then, high current density is passed through the anodes and cathode drum to produce continuously the copper foil. In this case, the surface of the foil which was in contact with the surface of the cathode drum, is the shiny side of the electrodeposited copper foil and the outer surface of the copper foil is the matte side.
The copper foil obtained through this electrolytic process is subjected to surface treatments. In these surface treatments, nodularization of the copper foil is performed for exerting an anchoring effect when bonding with a substrate, followed by zinc plating, chromating or silane coupling treatments for exerting a passivation effect. Finally, drying is performed to obtain the electrodeposited copper foil for making printed circuits.
On the other hand, in case of as-rolled copper foil, both surface sides of the copper foil are shiny or smooth. One side or both sides of these shiny sides is subjected to a surface treatment.
The copper foil having undergone the above surface treatments, because, for example, the electrolyte adheres to the surface thereof, must be washed with water (not shown) prior to the drying by means of a dryer for removing water from the surface of copper foil.
Therefore, it is common practice to perform drying of the electrolytic copper foil. This drying is generally accomplished by drying using hot air or using far infrared rays. The current situation is that drying by these methods is to about such an extent that the water adhering to the surface of the copper foil is removed and, thus, the drying temperature is held at up to 100.degree. C.
Heating the surface of the copper foil to 100.degree. C. or higher, for example, causes the zinc of the plated zinc layer provided on the surface of the copper foil to diffuse into the copper foil so that a zinc-copper alloying (brass formation) is effected. As a result, the dezincing phenomenon, which is the leaching of zinc into an acid such as hydrochloric acid used in the formation of circuit pattern, does not occur, thereby enhancing the acid resistance. Further, according to the inventors' investigations, the higher the surface temperature of the copper foil, the greater the peel strength relative to the resin substrate, until a peel strength peak at about 130.degree. C. as shown in FIG. 3.
Drying using hot air enables heating the copper foil and regulating the temperature at 130.degree. C. or higher. However, this method relies on the heating (drying) through the heat transfer from hot air, so that the energy loss attributed to discharged hot air is large. Further, as shown in FIG. 5, hot air drying apparatus 700 requires heater 701, fan 702 and circulation path 704 including path 703 for discharging a large volume of exhaust gas containing steam outside the apparatus. Therefore, unfavorably, the size of the apparatus is large, the space required is large, and the cost is high.
On the other hand, drying using far infrared rays, the surface of the copper foil reflects almost about 97% or more of the far infrared rays whose wavelength range is from 4 to 1000 .mu.m (see pages 6 to 120 of American Institute of Physics Handbook) and, hence, exhibits low absorption of far infrared rays. Therefore, the energy loss is large, and the temperature of the surface of the copper foil cannot be readily increased. Accordingly, a multiplicity of far infrared ray irradiating units must be arranged for attaining temperatures of 130.degree. C. or higher, thereby resulting in disadvantages in terms of apparatus, power consumption and cost.